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Discussion Starter #1
Wondering if anyone else has had this issue and figured out how to fix it. I have a 2014 750LiX. When I have the climate control in SYNC mode the rear fan will randomly start blowing on high speed for about 5 seconds and then go back to where it was set. This continues indefinitely unless I take the system out of SYNC and turn the rear fan off. When the fan blows on high the rear screen shows the same low fan speed as the front, even though it is clearly blowing much higher. Any thoughts?
 

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Nuclear engineer
02/2012 X5 35d M57Y CPO 99K miles NOKIAN WR G3 12K miles
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What is your indication of higher fan speed? Air flow noise or fan speed icons?
 

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Nuclear engineer
02/2012 X5 35d M57Y CPO 99K miles NOKIAN WR G3 12K miles
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The fan speed icon does not change. I can hear the higher air flow.
You hear air velocity converted from blower air pressure by the (variable) ‘nozzles’ formed by the flap openings. Your air flaps are moving making smaller orifices that increase air velocity.
 

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Discussion Starter #5
You hear air velocity converted from blower air pressure by the (variable) ‘nozzles’ formed by the flap openings. Your air flaps are moving making smaller orifices that increase air velocity.
I don't think that is it. It blows normal, then hard for 5 seconds, then normal again. It happened every few minutes.
 

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When you press the SYNC button...what is being synched? Air temperature or fan velocity/speed? I believe it is synching air temperature. So if the setting is in auto sync for the car to have the same temp in front and rear...and the rear has it's own fan...if it requires higher velocity to keep the temperature setting the same as the front...then wouldn't it be working correctly?

If the front is set to 72°F...and the rear temp sensor is reading 68°F...would the rear fan increase fan speed to send a greater amount of heated air to the rear so that it would match the front temp setting of 72°F? If not...how would the rear catch-up to match the front air temperature? Hmmm?

Here's some info from the TIS about FKA control panel (rear hvac) vs the front's IHKA control panel: https://www.newtis.info/tisv2/a/en/...ear-cabin-automatic-air-conditioning/4qMcgo8v

And here's another more inclusive explanation of the front IHKA and rear FKA controls...it does state that there are TWO blowers & blower output stages. Take note of the section about BLOWER & BLOWER OUTPUT STAGES...it discusses how the rear FKA blower creates the "necessary air mass flow": https://www.newtis.info/tisv2/a/en/.../climate-control-temperature-control/2cktSL1F
 

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Discussion Starter #7
When you press the SYNC button...what is being synched? Air temperature or fan velocity/speed? I believe it is synching air temperature. So if the setting is in auto sync for the car to have the same temp in front and rear...and the rear has it's own fan...if it requires higher velocity to keep the temperature setting the same as the front...then wouldn't it be working correctly?

If the front is set to 72°F...and the rear temp sensor is reading 68°F...would the rear fan increase fan speed to send a greater amount of heated air to the rear so that it would match the front temp setting of 72°F? If not...how would the rear catch-up to match the front air temperature? Hmmm?
There is no Hmmmm. The sync is set to whatever the driver decides. Passenger front, and both rear are set to whatever the driver sets when in sync. Makes no sense why the rear suddenly goes to high fan.
 

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There is no Hmmmm. The sync is set to whatever the driver decides. Passenger front, and both rear are set to whatever the driver sets when in sync. Makes no sense why the rear suddenly goes to high fan.
I would suggest to check out the links I posted in my previous post. If you have FKZ...then that blower has a stage output that is tied to the temp sensor. So again, if the front is set at a specific temperature and you press the SYNC button, then the rear blower (since it has its own stage output) can increase air mass so that the SET TEMP (yes, from the SYNC button) can be reached.

It appears that SYNC (to you) means that the blowers work in sync...but the tech info appears to indicate that the synch is for the left/right front IHKA and left/right rear FKA to be synched for the same TEMPERATURE...not fan speed. The blowers (aka fan speed) will adjust to maintain what the synced air temp setting is.
 

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Discussion Starter #9
Thank you for your replies. I am still of the opinion that if sync is ON, then the entire vehicle (temp, fan and air location) are set to whatever the driver has set whether that be in AUTO mode or not. If I'm in SYNC mode and someone in the rear seat (or front passenger seat) changes any setting then SYNC mode is immediately turned off on its own. The good news is that the rear blower is no longer going higher on its own. Whatever gremlin caused the issue to surface is no longer present.
 
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